We studied the background dwarf nova of KIC 11412044 in the Kepler public data and identified it with GALEX J194419.33+491257.0. This object turned out to be a very active SU UMa-type dwarf nova having a mean supercycle of about 150 d and frequent no
rmal outbursts having intervals of 4-10 d. The object showed strong persistent signal of the orbital variation with a period of 0.0528164(4) d (76.06 min) and superhumps with a typical period of 0.0548 d during superoutbursts. Most of the superoutbursts were accompanied by a precursor outburst. All these features are unusual for this very short orbital period. We succeeded in detecting the evolving stage of superhumps (stage A superhumps) and obtained a mass ratio of 0.141(2), which is unusually high for this orbital period. We suggest that the unusual outburst properties are a result of this high mass ratio. We suspect that this object is a member of the recently recognized class of cataclysmic variables (CVs) with a stripped core evolved secondary which are evolving toward AM CVn-type CVs. The present determination of the mass ratio using stage A superhumps makes the first case in such systems.
We made a supplemental study of the superoutbursts and superhumps in SU UMa stars by using the recently released Kepler public data of V1504 Cyg and V344 Lyr. One of the superoutbursts in V1504 Cyg was preceded by a precursor normal outburst which wa
s well separated from the main superoutburst. The superhump first appeared during the descending branch of the precursor normal outburst and it continued into quiescence (the deep dip between the precursor and the main superoutburst), and it began to grow in amplitude with the growth of the main superoutburst after quiescence ended. A similar phenomenon was also observed in V344 Lyr. This observation demonstrates very clearly that the superoutburst was triggered by the superhump (i.e., by the tidal instability), supporting the thermal-tidal instability model. Smak (2013, Acta Astron., 63, 109, arXiv:1301.0187) criticized our previous paper (Osaki and Kato, 2013, PASJ, 65, 50, arXiv:1212.1516) and challenged our main conclusion that various observational lines of evidence of V1504 Cyg support the thermal-tidal instability model for the superoutbursts of SU UMa stars. We present our detailed accounts to all of his criticisms by offering clear explanations. We conclude that the thermal-tidal instability model is after all only the viable model for the superoutbursts and superhumps in SU UMa stars.
We analyzed the Kepler long cadence data of KIC 7524178 (=KIS J192254.92+430905.4), and found that it is an SU UMa-type dwarf nova with frequent normal outbursts. The signal of the negative superhump was always the dominant one even during the supero
utburst, in contrast to our common knowledge about superhumps in dwarf novae. The signal of the positive superhump was only transiently seen during the superoutburst, and it quickly decayed after the superoutburst. The frequency variation of the negative superhump was similar to the two previously studied dwarf novae in the Kepler field, V1504 Cyg and V344 Lyr. This is the first object in which the negative superhumps dominate throughout the supercycle. Nevertheless, the superoutburst was faithfully accompanied by the positive superhump, indicating that the tidal eccentric instability is essential for triggering a superoutburst. All the pieces of evidence strengthen the thermal-tidal instability as the origin of the superoutburst and supercycle, making this object the third such example in the Kepler field. This object had unusually small (~1.0 mag) outburst amplitude and we discussed that the object has a high mass-transfer rate close to the thermal stability limit of the accretion disk. The periods of the negative and positive superhumps, and that of the candidate orbital period were 0.07288 d (average, variable in the range 0.0723-0.0731 d), 0.0785 d (average, variable in the range 0.0772-0.0788 d) and 0.074606(1) d, respectively.
We propose a new dynamical method to estimate binary mass ratios by using the period of superhumps in SU UMa-type dwarf novae during the growing stage (the stage A superhumps). This method is based on a working hypothesis in which the period of the s
uperhumps at the growing stage is determined by the dynamical precession rate at the 3:1 resonance radius, a picture suggested in our new interpretation of the superhump period evolution during the superoutburst (Osaki, Kato 2013, arXiv:1305.5877). By comparison with the objects with known mass ratios, we show that our method can provide sufficiently accurate mass ratios comparable to those obtained by quiescent eclipse observations. This method is very advantageous in that it requires neither eclipses, nor an experimental calibration. It is particularly suited for exploring the low mass-ratio end of the evolution of cataclysmic variables, where the secondary is undetectable by conventional methods. Our analysis suggests that previous estimates of mass ratios using superhump periods during superoutburst were systematically underestimated for low mass-ratio systems and we provided a new calibration. It suggests that most of WZ Sge-type dwarf novae have secondaries close to the border of the lower main-sequence and brown dwarfs, and most of the objects have not yet reached the evolutionary stage of period bouncers. Our result is not in contradiction with an assumption that the observed minimum period (~77 min) of ordinary hydrogen-rich cataclysmic variables is indeed the period minimum. We highlight the importance of early observation of stage A superhumps and propose a future desirable strategy of observation.
We have studied the short-cadence Kepler public light curves of SU UMa stars, V344 Lyr and V1504 Cyg extending over a period of more than two years by using power spectral analysis. We determined the orbital period of V344 Lyr to be Porb=0.087903(1)
d. We also reanalyzed the frequency variation of the negative superhump in a complete supercycle of V1504 Cyg with additional data of the O-C diagram, confirming that its characteristic variation is in accordance with the thermal-tidal instability model. We present a new two-dimensional period analysis based on a new method of a least absolute shrinkage and selection operator (Lasso). The new method gives very sharp peaks in the power spectra, and it is very useful for studying of the frequency variation in cataclysmic variable stars. We also analyzed simultaneous frequency variations of the positive and negative superhumps. If they are appropriately converted, it is found that they vary in unison, indicating that they represent a disk-radius variation. We have also studied the frequency (or period) variations of positive superhumps during superoutbursts. These variations can be understood in a qualitative way by combining of the disk radius variation and the variation of pressure effects during a superoutburst. A sudden excitation of oscillation with a frequency range near to the negative superhump (which we call impulsive negative superhump) was observed in the descending branch of several outbursts of V344 Lyr. These events seem to have occurred just prior to the next superoutburst, and to act as a lead of the impending superoutburst.
We studied the Kepler light curves of three SU UMa-type dwarf novae. Both the background dwarf nova of KIC 4378554 and V516 Lyr showed a combination of precursor-main superoutburst, during which superhumps always developed on the fading branch of the
precursor. This finding supports the thermal-tidal instability theory as the origin of the superoutburst. A superoutburst of V585 Lyr recorded by Kepler did not show a precursor outburst and the superhumps developed only after the maximum light, a first example in the Kepler data so far. Such a superoutburst is understood within the thermal-tidal instability model. The observation of V585 Lyr made the first clear Kepler detection of the positive period derivative commonly seen in the stage B superhumps in dwarf novae with short orbital periods. In all objects, there was no strong signature of a transition to the dominating stream impact-type component of superhumps, suggesting that there is no strong indication of an enhanced mass-transfer following the superoutburst. We have determined the orbital period of V516 Lyr to be 0.083999(8) d. In V516 Lyr, some of outbursts were double outbursts in a various degree. The preceding outburst in the double outburst was of the inside-out nature while the following one was of the outside-in nature. One of superoutbursts in V516 Lyr was preceded by a double precursor. The preceding precursor failed to trigger a superoutburst and the following precursor triggered a superoutburst by developing positive superhumps.
